1. Field of the Invention
It is an object of the present invention to provide new interferometers, which are better than prior art in respect to stability, scan speed and cost of manufacture. It is an object of the present inventions to improve the state-of-the-art in photometric accuracy of interferometric measurements.
2. Background Information
The book by Griffiths and deHaseth, “Fourier Transform Infrared Spectrometry,” ISBN 0-471-09902-3, is included by reference for the entirety of its content. This book describes much of the prior art and practice in great detail.
Michelson interferometers can be used for many purposes, including spectrometry and metrology. The principle of operation is that a beam of electromagnetic radiation or energy is divided into two portions, which propagate in first and second paths; the two portions are delayed and recombined, leading to interference that is a function of the path difference between the first and second paths, as well as frequency content of the radiation. This prior art is illustrated by FIG. 1. A source of radiation is indicated by 10 and the radiation may be collimated by a parabolic mirror 11 into a primary beam of energy. The primary beam is divided at the beamsplitter 30 into two portions, which propagate in first 15 and second 16 paths. These paths 15 and 16 are often called the arms of the interferometer, and are typically oriented at 90 degrees to each other, as shown in FIG. 1.
At the ends of the two arms or paths 15 and 16 are mirrors 80A and 80B from which the two beams are reflected back toward the beamsplitter, retracing more or less exactly paths 15 and 16. At the beamsplitter, each of the two returning beams are split again resulting in two recombined beams. One recombined beam propagates back toward the source, generally being lost from use, and the second (hereafter, “the recombined beam”) propagates out of the interferometer at an angle to the input beam, and follows a path to a detector. The second recombined beam may propagate to a parabolic focusing mirror 21A that concentrates the radiation at a sampling point 23. The radiation from the sampling point may be collected by a mirror 21B and focused on a detector 20. Many alternatives to the mirror combination 21A and 21B are known in the art. Radiation from a second source 12, generally having a precisely known wavelength, may be used as an internal standard of distance for the interferometer. Such a source 12 is often a helium-neon laser, but may be instead a diode laser, stabilized diode laser or gas discharge lamp. The radiation from the second source may be observed simultaneously with a second detector 21. External focusing optics generally are not required for a reference laser such as 12 because the beam is already tightly collimated. The interferometer usually is operated by moving one of the mirrors; the most common method for driving the mirror is a voice coil linear motor, but many other approaches are possible and some are known. The mirror may be moved at constant velocity and reciprocated, or it may be moved incrementally and stopped. It is understood that the signals from the detectors 12 and 22 are generally digitized and input to a computer. The data are processed by known means to generate and display spectra of the source radiation as modified by interaction with a sample.
A disadvantage of the Michelson interferometer and many related designs is that the two end mirrors 80A and 80B are susceptible to misalignment with each other and with the beamsplitter 30. Further, the alignment must be preserved, to interferometric tolerances, during and between motion of one or both of the mirrors 80A and 80B. Expensive, high-quality bearings are often required to provide such precise rectilinear motion. In some instruments, airbearings are used with the aforementioned voice coil linear motors to provide such motion. Various other solutions to this problem have been proposed in the literature and prior art. For example, Jamin (1856), Solomon (U.S. Pat. No. 5,675,412), Turner and Mould (U.S. Pat. No. 5,808,739), Spanner (U.S. Pat. No. 6,369,951), Frosch (U.S. Pat. No. 4,278,351), Woodruff (U.S. Pat. No. 4,391,525), and related designs provide interferometers in which slight misalignments of the optical components are compensated with respect to interferometric alignment. Slight residual misalignment may result in a beam of radiation not reaching exactly the intended detection location. The sensitivity to the resulting misalignment of the source image on the detector is roughly 100 times smaller than the sensitivity to interferometric misalignment. In short, there is a substantial advantage to optical tilt compensation. However, a disadvantage of the prior art is the long optical paths required for optical tilt compensation. For example, the approaches of Frosch, Woodruff, Solomon, Turner, and others, impose an undesirable limit on throughput, which is the product of solid angle and aperture area.
Larsson has described in U.S. Pat. No. 5,650,848 a method for tilt-compensating the scanning mirrors of an interferometer. This prior art extends ideas described by Steel in the 1960's (see for example, “Interferometry,” Cambridge University Press, 1967 W. H. Steel). Larsson's approach employs three mutually perpendicular facets on a common assembly, but the radiation reaches them in a substantially different order than in the present invention. The radiation first reaches a beamsplitter. After being divided into two beams, in Larsson's interferometer design, each of the two beams reaches a single reflector surface rigidly and perpendicularly mounted to the beamsplitter. Because the single reflectors are mutually perpendicular, the two beams from the beamsplitter are both folded by 90 degrees (in different planes), rendering them antiparallel. Two plane mirrors held in opposition on a common carriage may be used to return the beams to the beamsplitter. One substantial improvement, relative to Larsson, of the present invention described herein, is that two reflector facets are placed in one arm of an interferometer. Further, the facets are combined into a roof reflector, which makes the assembly of the present invention more compact, more rigid and provides a rigid mounting surface for the beamsplitter and associated components.
Bleier et al. describe what they term “monolithic optical assemblies” in two U.S. patent, U.S. Pat. No. 5,949,543 (1999), and U.S. Pat. No. 6,141,101 (2000). Their system might also be called a bilithic interferometer, in that two assemblies are juxtapositioned to form a variation of Michelson's design. The two assemblies are a hollow cube corner retroreflector and the monolithic assembly containing a beamsplitter and two reflecting surfaces. The two reflecting surfaces are interferometer end mirrors analogous to mirrors 80A and 80B of FIG. 1. Bleier's two reflecting surfaces, like 80A and 80B of FIG. 1, are mutually perpendicular with each other, but not with the beamsplitter; they are oriented at 45 degree angles relative to the beamsplitter. Hence, Bleier's monolithic interferometer is not tilt-compensated to the full extent of the present invention. However, the construction techniques taught by Bleier for producing monolithic assemblies can be applied to the interferometer designs of the present invention. This is not presently the preferred approach to construction, because the resulting assemblies are more fragile and thought to be more expensive than the construction of the preferred embodiments described below.